The present invention pertains to ultraviolet curable high molecular weight aqueous latex coatings particularly suitable for coil coating, metal decorator coating, wood base coatings, and similar industrial latex coatings.
A wide variety of ultraviolet curable coatings containing suitable ultraviolet sensitizers for initiating polymerization of ethylenically unsaturated polymers have been suggested wherein the ethylenic polymer double bonds cross-link upon exposure to a broad spectrum of ultraviolet wave energy generally between about 1,600 A and 4,000 A. Although latex coatings containing ethylenic unsaturation can be cured by ultraviolet energy, latex coatings cured by ultraviolet energy often are deficient in quality due to shrinkage of latex polymer in latex coatings which invariably produce films that are deficient in adhesion, flow-out, and leveling. Low molecular weight polymers cross-link with ultraviolet energy and exhibit good hardness and solvent resistance but are very brittle and not flexible with very poor adhesion. The inclusion of a thermoplastic deteriorates the film properties and solvent resistance. A high molecular weight polymer having reactive unsaturation exhibits very poor cure upon exposure to ultraviolet energy.
In the past, high molecular weight latex polymer particles were necessary to provide suitable film physical properties such as hardness, wear resistance, and other desirable film integrity properties to sufficiently protect the substrate and ordinarily were heated at high temperatures for time sufficient to obtain adequate flow-out and leveling of the latex particles. The high molecular weight latex particles must be maintained heated and fluid while irradiating with ultraviolet energy so as to obtain free radical propagation within the fluid film while the film coating is maintained in the fluid state. Fluidity is necessary to obtain efficient free radical propagation and thus obtain cross-linking of the ethylenic double bonds.
It now has been found that substantially improved ultraviolet curable latex coatings can be obtained by providing a high molecular weight primary latex binder in combination with an emulsified low molecular weight cross-linking oligomer or prepolymer adapted to cross-link the high molecular weight latex polymer particles upon exposure to ultraviolet energy. The high molecular weight primary binder is an ethylenically unsaturated polymer having a molecular weight preferably between about 100,000 and 1,000,000. The emulsified low molecular weight cross-linking polymer has molecular weight between about 300 and 3,000 and provides fluidity to the latex coating thereby eliminating the need for heating the coating to provide flow-out or leveling as well as eliminating the need for heating the coating during ultraviolet irradiation to maintain fluidity required for free radical propagation. The emulsified low molecular weight cross-linking polymer contains an ultraviolet sensitizer for rapidly initiating propagation of free radicals in the emulsified fluid phase which in turn substantially increases cross-linking with the high molecular weight primary binder latex. The low molecular weight cross-linker further can provide flexibility and adhesion to the substrate while still maintaining very desirable corrosion and wear resistance film characteristics of the primary high molecular weight polymer particles.
These and other advantages will become more apparent by referring to the detailed description of the invention.